The present invention relates to an image rejection mixer (IRM) for rejecting signals having image frequency and, more particularly, a mixer for rejecting signals of image frequency by using mismatch compensation.
In the super-heterodyne reception technique, the frequency as far as about two times of the intermediate frequency from the desired reception frequency in the frequency domain, is called the “Image Frequency.” Components having the image frequency, which is called the “Image Frequency Components” or the “Image Signal,” cause distortion of signals having intermediate frequency. Therefore, it is preferable to reject the image frequency components.
In order to reject image frequency components, band pass filtering method was used. By using a band pass filter to suppress image frequency components among signals received through an antenna, it was possible to prevent distortion of intermediate frequency signals caused by the image frequency components. However, the band pass filtering method costs high, since the cost to make a band pass filter is usually high.
As a method for rejecting image frequency components, image rejection mixer (IRM) architecture was suggested. The IRM architecture may be classified into the Hartley and the Weaver architectures. The Weaver architecture requires at least four mixers. The Weaver architecture, however, may effectively reject image frequency components throughout a band, which is wider than the band through which the Hartley architecture may reject image frequency components.
FIG. 1 shows a schematic view of a block diagram illustrating an IRM of the Weaver architecture. FIG. 2 shows a schematic view of the block diagram of FIG. 1 for illustrating the operation of the IRM of FIG. 1.
As shown in FIG. 1, an input radio frequency signal (RFIN) is mixed with the first local oscillation signal (LO1), cos(ωLO1t), at mixer M1. The frequency of the first local oscillation signal (LO1) is called the first local oscillation frequency (ωLO1). The input radio frequency signal (RFIN) is also mixed with the signal, sin(ωLO1t), which is resulted by shifting the first local oscillation signal (LO1) by −90°, at mixer M2. The outputs of mixers M1 and M2 are in-phase (I) and quadrature-phase (Q) signals, respectively. Frequency of the in-phase and quadrature-phase signals corresponds to the difference between frequency of the input signal and the first local oscillation frequency (ωLO1). The difference between the frequency of the input signal and the first local oscillation frequency (ωLO1) is called the first intermediate frequency (ωIF1).
The outputs of mixers M1 and M2 are provided to mixers M3 and M4, and mixers M5 and M6, respectively.
The in-phase signal (I) output from mixer M1 is mixed with the second local oscillation signal (LO2), cos((ωLO2t), at mixer M3. The frequency of the second local oscillation signal (LO2) is called the second local oscillation frequency (ωLO2). The in-phase signal (I) output from mixer M1 is also mixed with the signal, sin(ωLO2t), which is resulted by shifting the second local oscillation signal (LO2) by −90°, at mixer M4. The outputs of mixers M3 and M4 are in-phase (II) and quadrature-phase (IQ) signals, respectively. Frequency of the in-phase and quadrature-phase signals (II and IQ) corresponds to the difference between the first intermediate frequency (ωIF1) and the second local oscillation frequency (ωLO2) The difference between the first intermediate frequency (ωIF1) and the second local oscillation frequency (ωLO2) is called the second intermediate frequency (ωLO2).
The quadrature-phase signal (Q) output from mixer M2 is mixed with the second local oscillation signal (LO2), cos(ωLO2t), at mixer M5. The quadrature-phase signal (Q) output from mixer M2 is also mixed with the signal, sin(ωLO2t), which is resulted by shifting the second local oscillation signal (LO2) by −90°, at mixer M6. The outputs of mixers M5 and M6 are in-phase (QI) and quadrature-phase (QQ) signals, respectively. Frequency of the in-phase and quadrature-phase signals (QI and QQ) equals the second intermediate frequency (ωIF2)
A subtraction means subtracts the output (QQ) of mixer M6 from the output (II) of mixer M3, to produce an I-path signal. An addition means adds the outputs (IQ and QI) of mixers M4 and M5, to produce a Q-path signal.
Referring to FIG. 2, the operation of the circuit of FIG. 1 when an image signal, cos(ωimaget), is input. The frequency of the image signal is called the image frequency (ωimage).
The image signal, cos(ωimaget), is mixed with the first local oscillation signal (LO1), cos(ωLO1t), at mixer M1. The image signal, cos(ωimaget) is also mixed with the signal, sin(ωLO1t), which is resulted by shifting the first local oscillation signal (LO1) by −90°, at mixer M2. Mixer M1 produces a signal, cos(ωd1t), having frequency (ωd1) which corresponds to the difference between the image frequency (ωimaget) and the first local oscillation frequency (ωLO1). Mixer M2 produces a signal, sin(ωd1t), which lags 90° behind the output of mixer M1.
The outputs of mixers M1 and M2 are provided to mixers M3 and M4, and mixers M5 and M6, respectively.
At mixer M3, the output from mixer M1, cos(ωd1t), is mixed with the second local oscillation signal (LO2), cos(ωLO2t). Mixer M3 produces a signal, cos(ωd2t), having frequency (ωd2) which corresponds to the difference between the frequency (ωd1) of the output of mixer M1 and the second local oscillation frequency (ωL2)
At mixer M4, the output from mixer M1, cos(ωd1t), is also mixed with the signal, sin((ωLO2t), which is resulted by shifting the second local oscillation signal (LO2) by −90°. Mixer M4 produces a signal, sin(ωd2t), having frequency which equals that of the output of mixer M3. The output of mixer M4 lags 90° behind the output of mixer M3.
At mixer M5, the output from mixer M2, sin(ωd1t), is mixed with the second local oscillation signal (LO2), cos(ωLO2t). Mixer M5 produces a signal, sin(ωd2t), having frequency (ωd2) which equals those of the outputs of mixers M3 and M4. The output of mixer M5 lags 90° behind the output of mixer M3.
At mixer M6, the output from mixer M2 is also mixed with the signal, sin((ωLO2t), which is resulted by shifting the second local oscillation signal (LO2) by −90°. Mixer M6 produces a signal, cos(ωd2t), having frequency which equals those of the outputs of mixers M3, M4, and M5. Phase of the output signal of mixer M6 equals that of the output signal of mixer M3.
As shown above, the output of mixer M3 is identical to that of mixer M6. Therefore, the outputs of mixers M3 and M6 can be canceled out each other, by subtracting one from the other using a subtraction mean (A1). Further, the output of mixer M4 has phase that is 180° different from that of the output of mixer M5. Therefore, the outputs of mixers M4 and M5 can also be canceled out each other, by adding together using an addition mean (A2).
The outputs of mixers M1 through M6 further contain a plurality of frequency components, which are resulted by multiplication of input signal and the local oscillation signals. Those frequency components may be filtered out by using low pass filters.
In order to cancel out the image signal from the output of the mixer circuit as completely as possible, it is preferable that each gain of mixers M1 through M6 be equal. It is also preferable that the local oscillation signals provided to mixers M1 and M2 have equal frequency. Further, it is preferable that the local oscillation signals provided to mixers M3 through M6 have equal frequency.
In the Weaver architecture, one of the dominant reasons for the incomplete cancellation of the image frequency components is that gain is not equal between mixers M1 and M2, and among mixers M3, M4, M5, and M6. In the specification, the difference of gain between mixers M1 and M2 and among mixers M3, M4, M5, and M6 is called the gain mismatch. Other reason is that the phase difference between local oscillation signals provided to mixers M1 and M2, M3 and M4, and M5 and M6 is not correctly 90°. The deviation of the phase difference from 90° is called the phase mismatch.
It is known in the art that, in order to cancel image frequency components of 30 to 40 dB, the gain and phase mismatch should be within 0.2 to 0.4 dB and 0° to 2°. It is also known in the art that it is not easy to meet these requirements merely by making or changing design for a circuit and modifying layout of a circuit.